A lubrication system of a gear unit comprises typically a lubrication pump that is arranged to circulate lubricant oil through a gear stage or gear stages of the gear unit and through the bearings of the gear unit. As the viscosity of the lubricant oil is strongly dependent on its temperature, the lubrication pump can be overloaded or even damaged if the lubricant oil is cold and thereby its viscosity is too high for being pumped by the lubrication pump. Especially in cold weather conditions, the lubricant oil has to be heated during a starting phase of a gear unit so as to make the lubricant oil capable of being pumped and capable of properly lubricating the gear stage or gear stages of the gear unit and the bearings of the gear unit.
A known technical solution is to connect an external heater and a pump to an oil sump or an oil tank of a gear unit, where the pump is configured to circulate the lubricant oil through the external heater. An inconvenience related to this technical solution is that a quite long time is typically needed to heat up a sufficient amount of the lubricant oil in the oil sump or in the oil tank, and furthermore a large amount of energy is required.
Publication US20090191060 describes a gear unit comprising a lubrication pump for circulating lubrication oil, a suction pipe for transporting the lubrication oil from an oil sump or an oil tank to the lubrication pump, and a heating element in thermal connection with at least a portion of the suction pipe. The heating element is used to heat up the lubrication oil contained within the suction pipe to temperature where damaging of the lubrication pump is avoided. An inconvenience related to this approach is the difficulty to suck the lubricant oil through the suction pipe from the oil sump or the oil tank when the lubricant oil in the oil sump or in the oil tank is cold and thereby the viscosity of the lubricant oil in the oil sump or in the oil tank is high.
One known technical solution is to provide an oil sump or an oil tank with one or more immersion heaters for heating up the lubrication oil being in the oil sump or in the oil tank. An inconvenience related to this approach is that the heating power density, W/cm2, on the surface of the one or more immersion heaters has to be limited in order to avoid damaging the lubricant oil due to local overheating of the lubricant oil on the surface of the one or more immersion heaters. Limiting the heating power density increases, however, the time needed for heating up a sufficient amount of the lubricant oil to temperature where damaging of the lubrication pump is avoided.